As a reinforcement for a rubber product such as a rubber belt, tire and the like, reinforcing fibers such as glass fibers, polyester fibers, polyamide fibers and the like are widely used.
Since a rubber product such as a rubber belt and the like undergoes flex stress repeatedly, its performance degrades due to its flex fatigue, whereby a reinforcing fiber may be separated from a rubber matrix or worn out, with the result that its strength lowers easily. Such a phenomenon tends to be accelerated by heat and moisture, in particular. To prevent such flex fatigue by separation and obtain a sufficient reinforcing effect, drape and adhesion between a reinforcing fiber and rubber must be increased, and heat resistance and water resistance must be provided to the reinforcing fiber. To this end, various treatment agents are coated on the surface of a reinforcing fiber.
JP-A 1-221433 proposes various treatment agents such as a treatment agent comprising a resorcin-formaldehyde water-soluble condensate, vinylpyridine-butadiene-styrene terpolymer latex, dicarboxylated butadiene-styrene copolymer latex and chlorosulfonated polyethylene latex in combination.
Although the use of these treatment agents can improve adhesion between a reinforcing fiber and a rubber matrix and the heat resistance and flexing resistance of a treatment agent itself to a certain degree, it cannot yet be said that they are satisfactory. Therefore, rubber products having excellent heat resistance, water resistance and resistance to flex fatigue are difficult to obtain when reinforcing fibers treated with these treatment agents are used.
To increase adhesion between the reinforcing fiber and a rubber matrix, JP-B 5-71710 also proposes a rubber-reinforcing fiber formed by laminating the second layer containing a halogen-containing polymer and isocyanate on the first layer formed on a reinforcing fiber and containing a resorcin-formaldehyde water-soluble condensate and a latex and further laminating the third layer containing the same rubber as matrix rubber on the second layer. The production process of the rubber-reinforcing fiber of this type, however, is complicated and uneconomical since the second and third layer-forming steps are required, subsequently to the first layer-forming step.